LacusCurtius:
a large site on Roman antiquity, including a photosampler of Roman and Etruscan cities and monuments (with a very large site on
the city of Rome
of course); a site for teaching yourself to read Latin inscriptions; the complete Latin texts of Pliny the Elder's Natural History, Quintus Curtius' Histories of Alexander the Great, the Saturnalia of Macrobius, and Censorinus' de Die Natali; Suetonius, the Historia Augusta, Vitruvius, Claudian, Frontinus, Velleius Paterculus, Celsus, and Cato's de Re Rustica in both Latin and English; complete English translations of Caesar, Plutarch's Lives, Polybius, Cassius Dio, Appian's Civil Wars, Dio Chrysostom, Dionysius of Halicarnassus, Quintilian, and Oppian; several complete Greek texts in the original Greek; Rodolfo Lanciani's book Pagan and Christian Rome, Christian Hülsen's book on the Roman Forum, Bury's 2‑vol. History of the Later Roman Empire, Bevan's House of Ptolemy, 4 books on Roman Britain, George Dennis's Cities and Cemeteries of Etruria; Platner and Ashby's Topographical Dictionary of Ancient Rome (nearly complete) and most of Smith's Dictionary of Greek and Roman Antiquities; about 45% of Plutarch's Moralia; some maps of the Roman Empire, and lots more.

A bare index to the books onsite — just the books, though well over 100 of them — is available
here.

In a different category, one pretty specialized item, but for some few people it should be very useful, and it's free for the downloading:
Polytonic Greek Typinator Set
— a timesaving utility for anyone inputting a lot of ancient Greek.

After September 11, like many other Americans, I found myself drawn to the history of my own country; and as my small wartime contribution, I started an
American History
site, which has turned into one of the larger ones on the Web. Subsites on American Naval History
(15 complete books currently onsite), American Railroad History, and American Catholic History, several books on West Point (plus over 3000 entries from
Cullum's Register), and large sections on Illinois, Iowa, Louisiana, and North Carolina are joined by Freeman's biography of Robert E. Lee, books and articles on the history of a number of other States, a book on Washington's presidency and one on Wilson's, a contemporaneous account of the Baltimore Riot of 1812, a book on the San Francisco earthquake of 1906, the log kept by the Spanish commander at the siege of Pensacola in 1781, the journal of a Mormon pioneer, journal articles on a variety of subjects, and many other items. More is on its way.

A bare index to the books onsite — just the books, though well over 100 of them — is available
here.

The
History of the Americas
section is of course hardly an appendage to United States history, but the other way round; still, I'm a North American, so we can expect the broader part of the site to be smaller. Right now, Bourne's Spain in America,
Galdames'
History of Chile, and a large section on the
History of Brazil, including a full-length book on the subject in addition to a number of journal articles.

Sidelights on Dutch History
is a similar orientation page, with journal articles on Dutch maritime power and the 1667 invasion of England; an additional 50 webpages, 1235 pages of print, fall primarily under the history of the United States and of Brazil.

[ 2/13/14:
12 webpages
— 223 pages of print,
2 maps,
18 photos
]

My
Gazetteer of Italy
— currently over 1500 mostly non-Roman pages of churches, frescoes, etc. — is my own favorite part of the site. Since 2003, I've mostly been adding to the
Churches of Italy
section, which currently
(8/17/14) covers
694 churches in 394 pages and 1592 photos; plus, quite separately, three entire books on the churches of Rome, covering about 900 of them, past and present, in great detail; and several books covering many of the churches of Umbria and of the city of L'Aquila in the Abruzzo. (The merest drop in a bucket, by the way: Italy's churches present and past must number at least 500,000.)

The United States, my home, I know far less well than I do Italy: for one thing, they're a much larger country. My
American Scrapbook
for now
— 1/21/10 — is mostly about Kentucky (in particular the little town of
Jenkins), with a bit of Chicago.

Also, a few loose ends that will eventually be better organized; in roughly chronological order:

About 16 months' worth of my
diary.
Nothing terribly titillating, really; but it's the laid-back section of this website (read: "easy to put online"), and the raw material for much of the Gazetteer. A bit of London, France, and Kentucky, and lots of Italy: Rome, Milan, Tuscany, Umbria and the Marche, large tracts of which I explored on foot, so that the diary includes details that could be useful if you're planning a trip or a bike tour. Illustrated with photos not usually found elsewhere onsite, cross-linked to Gazetteer pages and external sites, and lavishly supplied with Google maps, it's also partly indexed by place and topic.